Pressure washers have been designed wherein a pump pressurizes a fluid which is ejected as a stream or spray out of a nozzle. In a floor-standing pressure washer, the pump is disposed in a floor standing unit to which a spray gun is connected by a hose and flow is controlled by a flow control valve disposed in the spray gun. In a hand-held pressure washer, a pump and valve are incorporated in a spray gun, which is connected to a fluid source by a hose. The flow of pressurized fluid out of the nozzle is selectively controlled by turning the pump on or off by means of a switch carried by the gun.
In the floor-standing embodiment, the pump operates continuously because no means for activating and deactivating the pump is integrated into the spray gun.
In either version of the pressure washer, it is desirable to provide a bypass valve which recirculates fluid flowing out of the pump back to the intake of the pump when the fluid is prevented from flowing out of the pressure washer through the spray nozzle, for example, when a blockage occurs in the fluid flow path. In addition, in at least the floor-standing embodiment, a limited flow of fluid out of the nozzle may be permitted so that the pump may be cooled by fresh (i.e., nonrecirculated) fluid. In this way, the pump can operate continuously without being subjected to undue stress and premature failure.
The pressure washer disclosed in Paige, et al., parent application Ser. No. 07/819,351, now U.S. Pat. No. 5,259,556, includes a bypass valve (hereinafter the "prior valve") in which a shuttle is moveable in a bypass chamber between a first position, in which the shuttle blocks the flow of fluid from the bypass chamber into a bypass conduit, and a second position, in which the shuttle permits such flow when fluid flow out of the valve is blocked. Paige, et al. '556 also discloses means for biasing the shuttle toward the second position to permit fluid to flow from the bypass chamber into the bypass conduit.
In the prior valve, a pair of fluid seals are carried by and circumferentially surround the shuttle. The seals are disposed in contact with the inner surfaces of the valve housing when the shuttle is in the first position to prevent flow of fluid out of the bypass chamber into the bypass conduit. When the shuttle moves to the second position to permit such flow, however, one of the fluid seals moves to a region of the valve housing having an inner diameter larger than the outer diameter of that seal. As a result, that seal moves out of contact with the inner surface of the valve housing, allowing debris to accumulate between the seal and the housing and permitting the seal to become misshapen. Thereafter, the debris or the seal itself may prevent the shuttle from being moved to the first position so that the bypass function is impaired.
In addition, loss of sealing contact results in a loss of the pressure-force differential that is used to move the shuttle to the rear position. Still further, once the pressure-force differential is lost, the frictional engagement of the seals surrounding the shuttle is likewise substantially lost, thereby permitting the movement of the shuttle in response to relatively small forces. Thereafter, when partial flow of fluid out of the spray nozzle occurs, such as when limited fluid flow out of the nozzle is permitted for cooling purposes, the shuttle tends to oscillate or otherwise act in an indeterminate manner, thereby impairing the bypass function. While a spring may be added to bias the shuttle rearward and reduce this indeterminate behavior, it has been found that this undesired effect cannot be eliminated entirely in the prior valve.